Lan Nguyen Chemistry Information class 2012 Dec 08, 2012
Developments of Ion Chromatography Technology with Various Suppressors Devices and without Suppressors
I) Introduction: Ion Chromatography (IC) has an important role in the analysis of inorganic and organic ions. The IC is developed to separate ions and detection methods in many difference ways [1]. This is only system using two columns before going to detector [4]. It uses to determine environmentally waters such as waste water, rivers, lakes and fluids of biological such as blood and urine [2]. More than twenty years ago, IC was only able to identify in inorganic ion, used ion exchange column to separate the ions, and detected by conductometric monitoring [1]. This paper will focus on the elution suppression, suppressors in modern ion chromatography and the application of IC.
II) Many types of Suppressors A) Stripper column or Suppressor In the past, Ion exchange resin column provided good separation of ions, but the eluent from this column carry on the background electrolyte that prevented the detector was not able to detect ions interest against this background. New stripper column combination with resins or suppressor was introduced to “strip out” or neutralized the background electrolyte ions [2]. The suppressor employed two important roles in ion chromatography. The first role improved detection sensitivity and other reduced the background of the eluent to enhance peaks of ions more sensitivity [3].
This new method process of IC was added stripper column or suppressor after separate column to neutralize electrolyte ions leaving background of de-ionized water. The interest ions were detected without interfere with ion of background. For example figure 1, the cation analysis samples containing Li+, Na+, and K+, using HCl as the eluant is passed to two columns separate column and stripper column. When cation sample is injected to first column, they will be separated and will exit with different times from the bottom of column. In the stripper column, there are two reactions occurred : 1) HCl is replaced by the strong base resin; 2) the chlorides are changed to hydroxides [2]. However, the stripper column caused the peak broader and loss resolution [4, 5].
Figure 1: Separation of alkali metal [4]
B) Hollow Fibers Suppressor
The ion-exchange hollow fibers suppressor was introduced to solve the problem of stripper column. The hollow fiber suppressor was made by a coiled of 2 mm i.d, 1.8 in. o.d., and 316 stainless steel (figure 2). This fiber suppressor used with sodium carbonate as the eluent and it was able to determine anions samples. The sulfuric acid was supplied while using the hollow fiber suppressor. The material of walls of this suppressor was the sulfonated polyethylene fiber to allow sodium ion going out and hydrogen ion going through the wall. Sulfate and carbonate ions did not pass the fiber wall. Carbonate ion attached to hydrogen ion to make carbonic acids which enhance conductivity of anion samples. Besides, sulfate ion attached to sodium ion to product sodium sulfate [4]. Figure 2: Theory of operation of the Hollow Fiber Suppressor [4].
The hollow fiber suppressor was compared to two suppressor columns to determine acetate ion ( figure 3). The small suppressor column had size 2.8 X 300 mm and the large one had size 9 X 110 mm. By using difference suppressors the acetate ion peak presented differences. The small suppressor column resulted peak of acetate ion sharp and tall, while the peak of large suppressor showed broaden and retard. The acetate peak of hollow fiber suppressor was taller and sharper than both suppressor columns. [4]
Figure 3: Chromatographic response to an injection of acetate with the use of suppressor columns and the hollow fiber suppressor [4].
On the other hand, hollow fiber suppressor also had negative effects. The rate of diffusion was limited in the hollow fiber. A fiber suppressor with tiny diameter and thin walls could drop the pressure during the fiber blockage by small gas bubbles and the fiber may be damaged by this problem.[5]
C) Micro-membrane Suppressor To continue improving suppressor, a new micro-membrane suppressor (MMS) was created to replace hollow fiber suppressor [5]. The advance of new MMS showed higher capacity, lower dead volume, and longer lifetime. The design of this suppressor contained two semi-permeable ion-exchange membranes in the middle of three sets of ion-exchange (figure 4). The ion-exchange screen stimulated the suppression reaction in very little volume flow. The screen was defined as eluent chamber. These membranes allowed the regenerating ion passing in and the eluent oppose ion going out of the eluent chamber, this processing similarly to hollow fiber suppressor [6]. The regeneration flowed to direction of outer surfaces of both these membranes making constant regeneration. This regeneration produced hydrogen or hydroxide ion when passing by a constant flow of a regenerate solution from outside source, or by electrolysis of water [3].
Figure 4: Internal design of the MicroMembrane Suppressor [6].
Membrane suppressors had higher suppression capacity. It allowed the flow of ions transfer thought membrane with high speed. The advance of membrane reduced low dead volume, improved suppression capacity and was able to handle high concentration of sodium hydroxide eluent. Figure 5 showed the example of anion analysis with good separate and good signal of peaks. This membrane suppressor also opened the step to gradient eluent for the conductormetric mode of ion chromatography [1]. The gradient eluent for IC was popularity to users. The problem of this eluent increased conductance and noise of background [12].
Figure 5: Ion chromatography of anions using micro-membrane suppressor [1]. Some the problems of these membrane suppressors occurred such as causing higher background and noise because of counter ion leakage to the eluent chamber [1], requiring a constant flow of regeneration and it generate a large amount of chemical waste, increasing cost of instrument, reducing reliability of analysis samples[6]. Membranes needed to replace every three to twelve months depending on application [3].
D) Ion-exchange Membrane Tubing Suppressor Ion-exchange membrane tubing suppressor (figure 6) was another type of membrane suppressor was introduced. It contained of two coaxial tubes. The inside of tube was made of perfluorosulphonic acid cation-exchange membrane by Dupont Nafion type 881 X. This material was heated to temperature at 150oC, and then stretched its length longer 3.75 times to make a tube of 0.4 mm I.D and 0.55 O.D. A length of this resultant tube was added coaxially into PTFE tube with I.D about 1 mm. Eluent and separated ionic flowed inside the tube, while scavenger (aqueous acid) flowed outside the tube. In the process, the cation- exchange membrane was soaked to cations but was resistant to anions, therefore cations in the eluent and samples were exchanged with hydrogen ion in the scavenger [7]. These suppressors contained cation-exchange membranes tubing which was able to run many analysis samples in the long time and reduced the board peaks effects. In the system, the separator column with small-anion latex was attached to styrene-divinylbenzene copolymer. The combination of separator column and suppressor provided good sharp of anion analysis samples. These suppressors also were available for many choice of eluent, the concentration of eluent, and flow rate of eluent without any problem effected to regeneration such as time dependence of water dip and nitrite height changes [7].
Figure 6: Theory of operation of a cation-exchange membrane suppressor. DBS-H=dodecylbenzenesulfonic acid; DBS-Na =sodium dodecylbenzenesulphonate [7].
The 200 ppm chloride ion sample was tested by passing tube suppressor without a separator column. Water source was the first flowing, and 0.05 M DBS used as scavenger flow. The noise of background conductance reduced from 1.5 mS/cm to 30 µs/cm after adding scavenger. The process conversion of carbonate and sodium bicarbonate to carbonic acid caused lower conductivity. At the same time, sodium chloride was transformed into hydrochloric to make the peak the chloride ion was higher 2.5 times (see Figure 7). The peak height of samples depended on the flow rate of scavenger and flow rate of eluent. The scavenger can be used as any kinds of aqueous acid as soon as using cation-exchange membrane. It was possible the leakage of scavenger into the eluent so the molecular weight of the scavenger should be large [7].
Figure 7: Effect of the tubing suppressor [7].
E) Solid-Phase Chemical Suppressors (SPCSs)
Following suppressors named solid-phase chemical suppressors (SPCSs). This SPCS used disposable pack-bed cartridges. It was protected better than membrane-based suppressor. The cartridge was not expensive so it could be discarded when effecting to signal of peaks. Using these suppressors reduced the maintenance time for regeneration system and avoided the complex post-column reaction system [5]. F) Electrochemically Regenerated Ion Suppressors (ERIS)
Based on the SPCS advance, an electrochemically regenerated ion suppressor (trade mark ERIS) was designed to simplify the way of taking care of suppressors during operation. The ERIS Autosuppressor needed the electrolysis of water to regenerate the solid-phase suppressor cells. Below the reaction of the electrolysis of water in the cells [3];
At the both electrodes produced hydrogen ions. One cell was used to suppress the eluent, while other cell was electrochemical suppressor cells helping cycling of the process when the instrument operating analyzed the samples [3]. At the end of each suppressor cell contained electrodes for reaction happening during cation and anion analyses. The hydrogen ion was created at the anode moving to packing sodium from resin converted back the hydrogen form during the electrochemical regeneration. Similarly, the hydroxide ion produced at the cathode was passed through the chloride form resin, changing it back to hydroxide form (see figure 8). By these processes, there were no requirement for regenerant reagents and no due with chemical waste. The problem of ERIS Autosuppressor was the suppressor cell was always finished regenerated before each injection [3]. The ERIS Autosuppressors operated at high pressure, and got along with electroactive eluents and organic containing eluents. The electrochemical regeneration avoided the need for regenerant reagent and equipment. This suppressor had two suppressor cells; one cell used for suppression, and other was regenerated and equilibrated. A switching valve was also need to continuous the operation [3, 8]
For cation analysis used hydrochloric acid as electroactive eluents did not damage the ERIS suppressor cells but it damaged in the membranes. The example of cation (see Figure 9) showed the well separation of cations ion with hydrochloride acid as eluent. Although the sample was run for two weeks without stopping, there was not deterioration in the suppressor cell was detected. However the studied showed the containing of methanol in eluents damaged cell after the electrolysis was done [3].
The advances of electrochemical type suppressor such as: (1) avoid a flowing scavenger solution ;(2) improve the suppression performance; 3) simplify operation procedure: (4) reduce back pressure of suppressor; (5) the suppressor last longer; and (6) decrease cost of suppressor [5].
Figure 8 : suppression and electrochemical regeneration during (a) anion analysis and (b) cation analysis. Cell “A” suppresses the eluent while cell “ B” electrochemically regenerated [3]. Figure 9: Separation of cations using hydrochloric acid eluent. Peak identification: 1=lithium (0.2 ppm), 2=sodium ( 1.5 ppm), 3= ammonium (1.5 ppm), 4= potassium (2.4 ppm), 5=magnesium ( 2.0 ppm), 6=calcium (2.0 ppm). Column universal cation ( 100 mmX 4.6 mm); eluent, 3 mM hydrochloric acid; flow-rate ,1.0 ml/ml; detection, suppressed conductivity. [3]
H) Electrolytic Suppressors (SRS) New electrolytic suppressor for IC was introduced after electrochemical suppressor. It named the “Self Regeneration Suppressor” or SRS. This new model designed with two platinum electrodes allowed electrodes of water goes through. The electrolytic reaction was generated by a constant controller electric field power. The design of the suppressor (see figure10) included ion-exchange screen to present in the regenerant chambers to help electric current. The ion-exchange membranes identified an eluent chamber that contained another ion-exchange screen to limit dead volume. Two platinum electrodes were inserted in the regenerant chamber; one was in the middle of the hardware shell and a regenerant screen, and other in the middle of the second regenerant screen and the membrane. The present of electrode employed high efficiency and eliminated gases of the waste. In the SRS suppressors showed the neutralization reaction same as in the membrane suppressors [6].
1) Anion Self Regenerating Suppressors( ASRSs) The SRS used for anion analysis called Anion Self Regenerating Suppressor (ASRS). At the anode hydrogen ion was created passing through the cation-exchange membrane to neutralize the sodium hydroxide or any basis eluent to form water. After neutralizing, the water moved to the detector cell. Positive charge like sodium was attracted to negatively charge cathode. The membrane in the cathode chamber absorbed sodium and released electrogenerated hydroxide ion to maintain electronic neutrality. The gases of hydrogen were generated from cathode and the gases of oxygen were generated from anode combined with the liquid waste of aqueous sodium hydroxide to the waste tubes [6]. 2) Cation Self Regenerating Suppressors ( CSRSs) The SRS used for cation analysis called Cation Self Regenerating Suppressor (CSRS). At the cathode hydroxide ion was generated going to anion-exchange membrane to neutralize the methanesulfonic acid or any acidic eluent to form water. This neutral eluent continued to the detector cell. Methanesulfonate ions was connected to the anode go to cathode membrane reactive with hydronium ions to endure electronic neutrality. Similarly to anion, the waste gases of neutralized processing and methanesulfonic acid went to the waste and vent [6].
The advance of SRS required the low flow-rates for the regenerant water. It used the deionized eluent from the detector cell waste as water source, excellent for reducing waste. SRS was able to handle high sample load and used the high ionic mobile phases. The good thing of an electrolytic suppressor had a low-resistance pathway using ion-exchange sites to flow between the electrodes allowing conductance of the electrical current [6].
There are three self-operational advances in the new electrolytic suppressor such as: 1) Cell effluent recycling: After the eluent went through the separation column, moved to the suppressor where the eluent was neutralized and removed the opposite ions. At this moment, the most the suppressed eluent was deionized ion and very few analyte ions went to the detector cell and came back to the suppressor as a routing. The advance of cell recycling reduced chemical waste cost of operation, and time to maintain suppressors. 2) External water source: the DI water used to increase the flow-rate through the regenerant chambers removing unwanted eluent counterions. This process helped to lower noise and more sensitivity. 3) Chemical regeneration mode: this new suppressor can be used with chemical regenerants as the membrane suppressors [6].
Figure 10: Internal schematic of the Self Regenerating Suppressor [6].
III) No Suppressor Column for Anion Chromatography
1) This system used a conductivity detector for anion chromatography without adding suppressors (see figure 11). It required a special anion-exchange with very low capacity and an eluent had low conductivity. It required a special anion-exchange resin with very low capacity and an eluent with low conductivity. There were three low-capacity anion-exchange were prepared in lab. [9]
The first one used copolymer (cross-linked polystyrene beads XAD-1) as starting material. This ion-exchange resin had capacity of 0.04 mequiv./g. and its mechanical was stable. Second, 3 g of zinc chloride were added to the mixture of 10 ml of chloromethyl methyl ether, 10 ml of methylene chlorid, 3 ml of nitromethane. The beads were formed after adding water to mixture and they used as ion-exchange resin with capacity of 0.07 mequiv./g. Third, the chloromethylated beads was made from liquefied trimethylamine with capacity 0.007 mequiv./g. An eluent with low conductivity used such as aqueous solution 5.0 *10-4 M of potassium benzoate, potassium phthalate or ammonium o-sulfobenzoate. The resin with low capacity and eluent low conductivity employed the sensitivity of anion chromatography at low concentration. The study showed the resins of lower capacity resulted lower detection limits [9].
The eluent anion was taken by the anion –exchange resin moving anion to be separated to chromatographic column. At low concentration of the eluent, the signals of anions were higher than the eluent background showing a positive peak. If the concentration of eluent greater than anions samples, the peaks of anions showed negative [10].
Figure 11: Separation of 4.8 ppm of fluoride, 5.1 ppm of chloride and 26.0 ppm of bromide. Resin, XAD-1, 0.04 mequiv./g; Eluent, 6.5* 10-4 M potassium benzoate, pH=4.6 [9].
(SO42-, Cl- , NO2- , Br- , and NO3-) were found in both samples with flowing concentration 42.1, 78.3, 0.81, 0.36 and 10.2 μM in snow and 20.1, 38.7, 0.41, 0.17, and 5.2 Mμmin rainwater. The retention time of samples were changed depending on the concentration of eluent [14].
The liquids samples from a drill stem test (DST) containing reservoir brine was found 5.5 ppm lithium, 58170 ppm sodium, 147 ppm ammonium and 1353 ppm potassium. The second DST sample containing reservoir brine and potassium chloride-based drilling mud was found 3.2 ppm lithium, 33570 ppm sodium, 65 ppm ammonium and 7980 ppm potassium. The study also identifies that hydrochloric acid does not elute cation earth metals [15].
Alkanli an alkaline earth ions are analyzed by IC to be able separated. The samples contain 1.0 ppm lithium, 9700 ppm sodium, 98 ppm ammonium, 63 ppm potassium, 85 ppm magnesium, 152 calcium, 17.6 ppm strontium, 1.0 ppm barium. This method used an IonPac CS12A as the separation column and H2SO4 as eluent. This is very useful method to analyze cation ions [16].
IV. In Conclusion:
Ion Chromatography can analyze with or without suppressors presentment. However, the report showed that suppressors have better sensitivity and more dynamic of peaks ions than without suppressors. The growing of ion chromatography instrument developed in commercially a decade ago. IC is a mature instrument with good sensitivity. It is able to analysis both cations and anions and easy to maintain [13]. The studiers continue research and development to respond to the analytical challenges of ion chromatography [1]. Ion Chromatography is advance instruments for analyzing earth metals and useful to determine environmentally waters [4].
References: 1) Chaga, G. Twenty-five years of immobilized metal ion affinity chromatography: past, present and future. J. Biochem. Biophys. Methods2001, 49, 313-334.DOI 2) Small, H.; Stevens, T. S.; Bauman, W. C. Novel ion exchange chromatographic method using conductimetric detection. Anal. Chem. 1975, DOI 3) SaariNordhaus, R.; Anderson, J. Electrochemically regenerated ion suppressors, new suppressors for ion chromatography. Journal ofChromatography a 1997, 782, 75-79.DOI 4) Stevens, T.; Davis, J.; Small, H. Hollow Fiber Ion-Exchange Suppressor for Ion Chromatography. Anal. Chem. 1981, 53, 1488-1492.DOI 5) TIAN, Z.; HU, R.; LIN, H.; WU, J. High-Performance Electrochemical Suppressor for Ion Chromatography. J. Chromatogr. 1988, 439, 159-163.DOI 6) Rabin, S.; Stillian, J.; Barreto, V.; Friedman, K.; Toofan, M. New Membrane-Based Electrolytic Suppressor Device for Suppressed Conductivity Detection in Ion Chromatography. J. Chromatogr. 1993, 640, 97-109.DOI 7) Hanaoka, Y.; Murayama, T.; Muramoto, S.; Matsuura, T.; Nanba, A. Ion Chromatography with an Ion-Exchange Membrane Suppressor. J. Chromatogr. 1982, 239, 537-548.DOI 8) Saari-Nordhaus, R.; Anderson, J. Recent advances in ion chromatography suppressor improve anion separation and detection. Journal of Chromatography a 2002, 956, 15-22 DOI (9) Gjerde, D.; Schmuckler, G.; Fritz, J. Anion Chromatography with Low-Conductivity Eluents .2. J. Chromatogr. 1980, 187, 35-45. DOI (10)Gjerde, D.; Schmuckler, G.; Fritz, J. Anion Chromatography with Low-Conductivity Eluents .2. J. Chromatogr. 1979, , 509-519. DOI (11) Rapsomnikis, S.; Harrison, R. Optimization of Single-Column Anion Chromatography with Indirect Ultraviolet Photometric and Fluorimetric Detection. Anal. Chim. Acta 1987, 199, 41-47 DOI (12) Noble, D. Ion Chromatography, Anal. Chem., 67 (1995), p. 205A DOI (13) Kadnar, R. Determination of alkali and alkaline earth metals in oilfield waters by ion chromatography. Journal of Chromatography a1998, 804, 217-221. DOI
(14) Hu, W.; Tanaka, K.; Haddad, P.; Hasebe, K. Suppressed electrostatic ion chromatography with tetraborate as eluent and its application to the determination of inorganic anions in snow and rainwater. Journal of Chromatography a 2000, 884, 161-165. DOI (15) Morales, J.; Demedina, H.; Denava, M.; Velasquez, H.; Santana, M. Determination of Organic-Acids by Ion Chromatography in Rain Water in the State of Zulia, Venezuela. Journal of Chromatography a 1994, 671, 193-196.DOI (16) Maurino, V.; Minero, C. Cyanuric acid-based eluent for suppressed anion chromatography. Anal. Chem. 1997, 69, 3333-3338.DOI
Chemistry Information class 2012
Dec 08, 2012
Developments of Ion Chromatography Technology with Various Suppressors Devices and without Suppressors
I) Introduction:
Ion Chromatography (IC) has an important role in the analysis of inorganic and organic ions. The IC is developed to separate ions and detection methods in many difference ways [1]. This is only system using two columns before going to detector [4]. It uses to determine environmentally waters such as waste water, rivers, lakes and fluids of biological such as blood and urine [2]. More than twenty years ago, IC was only able to identify in inorganic ion, used ion exchange column to separate the ions, and detected by conductometric monitoring [1]. This paper will focus on the elution suppression, suppressors in modern ion chromatography and the application of IC.
II) Many types of Suppressors
A) Stripper column or Suppressor
In the past, Ion exchange resin column provided good separation of ions, but the eluent from this column carry on the background electrolyte that prevented the detector was not able to detect ions interest against this background. New stripper column combination with resins or suppressor was introduced to “strip out” or neutralized the background electrolyte ions [2]. The suppressor employed two important roles in ion chromatography. The first role improved detection sensitivity and other reduced the background of the eluent to enhance peaks of ions more sensitivity [3].
This new method process of IC was added stripper column or suppressor after separate column to neutralize electrolyte ions leaving background of de-ionized water. The interest ions were detected without interfere with ion of background. For example figure 1, the cation analysis samples containing Li+, Na+, and K+, using HCl as the eluant is passed to two columns separate column and stripper column. When cation sample is injected to first column, they will be separated and will exit with different times from the bottom of column. In the stripper column, there are two reactions occurred : 1) HCl is replaced by the strong base resin; 2) the chlorides are changed to hydroxides [2]. However, the stripper column caused the peak broader and loss resolution [4, 5].
Figure 1: Separation of alkali metal [4]
B) Hollow Fibers Suppressor
The ion-exchange hollow fibers suppressor was introduced to solve the problem of stripper column. The hollow fiber suppressor was made by a coiled of 2 mm i.d, 1.8 in. o.d., and 316 stainless steel (figure 2). This fiber suppressor used with sodium carbonate as the eluent and it was able to determine anions samples. The sulfuric acid was supplied while using the hollow fiber suppressor. The material of walls of this suppressor was the sulfonated polyethylene fiber to allow sodium ion going out and hydrogen ion going through the wall. Sulfate and carbonate ions did not pass the fiber wall. Carbonate ion attached to hydrogen ion to make carbonic acids which enhance conductivity of anion samples. Besides, sulfate ion attached to sodium ion to product sodium sulfate [4].
Figure 2: Theory of operation of the Hollow Fiber Suppressor [4].
The hollow fiber suppressor was compared to two suppressor columns to determine acetate ion ( figure 3). The small suppressor column had size 2.8 X 300 mm and the large one had size 9 X 110 mm. By using difference suppressors the acetate ion peak presented differences. The small suppressor column resulted peak of acetate ion sharp and tall, while the peak of large suppressor showed broaden and retard. The acetate peak of hollow fiber suppressor was taller and sharper than both suppressor columns. [4]
Figure 3: Chromatographic response to an injection of acetate with the use of suppressor columns and the hollow fiber suppressor [4].
On the other hand, hollow fiber suppressor also had negative effects. The rate of diffusion was limited in the hollow fiber. A fiber suppressor with tiny diameter and thin walls could drop the pressure during the fiber blockage by small gas bubbles and the fiber may be damaged by this problem.[5]
C) Micro-membrane Suppressor
To continue improving suppressor, a new micro-membrane suppressor (MMS) was created to replace hollow fiber suppressor [5]. The advance of new MMS showed higher capacity, lower dead volume, and longer lifetime. The design of this suppressor contained two semi-permeable ion-exchange membranes in the middle of three sets of ion-exchange (figure 4). The ion-exchange screen stimulated the suppression reaction in very little volume flow. The screen was defined as eluent chamber. These membranes allowed the regenerating ion passing in and the eluent oppose ion going out of the eluent chamber, this processing similarly to hollow fiber suppressor [6]. The regeneration flowed to direction of outer surfaces of both these membranes making constant regeneration. This regeneration produced hydrogen or hydroxide ion when passing by a constant flow of a regenerate solution from outside source, or by electrolysis of water [3].
Figure 4: Internal design of the MicroMembrane Suppressor [6].
Membrane suppressors had higher suppression capacity. It allowed the flow of ions transfer thought membrane with high speed. The advance of membrane reduced low dead volume, improved suppression capacity and was able to handle high concentration of sodium hydroxide eluent. Figure 5 showed the example of anion analysis with good separate and good signal of peaks. This membrane suppressor also opened the step to gradient eluent for the conductormetric mode of ion chromatography [1]. The gradient eluent for IC was popularity to users. The problem of this eluent increased conductance and noise of background [12].
Figure 5: Ion chromatography of anions using micro-membrane suppressor [1].
Some the problems of these membrane suppressors occurred such as causing higher background and noise because of counter ion leakage to the eluent chamber [1], requiring a constant flow of regeneration and it generate a large amount of chemical waste, increasing cost of instrument, reducing reliability of analysis samples[6]. Membranes needed to replace every three to twelve months depending on application [3].
D) Ion-exchange Membrane Tubing Suppressor
Ion-exchange membrane tubing suppressor (figure 6) was another type of membrane suppressor was introduced. It contained of two coaxial tubes. The inside of tube was made of perfluorosulphonic acid cation-exchange membrane by Dupont Nafion type 881 X. This material was heated to temperature at 150oC, and then stretched its length longer 3.75 times to make a tube of 0.4 mm I.D and 0.55 O.D. A length of this resultant tube was added coaxially into PTFE tube with I.D about 1 mm. Eluent and separated ionic flowed inside the tube, while scavenger (aqueous acid) flowed outside the tube. In the process, the cation- exchange membrane was soaked to cations but was resistant to anions, therefore cations in the eluent and samples were exchanged with hydrogen ion in the scavenger [7].
These suppressors contained cation-exchange membranes tubing which was able to run many analysis samples in the long time and reduced the board peaks effects. In the system, the separator column with small-anion latex was attached to styrene-divinylbenzene copolymer. The combination of separator column and suppressor provided good sharp of anion analysis samples. These suppressors also were available for many choice of eluent, the concentration of eluent, and flow rate of eluent without any problem effected to regeneration such as time dependence of water dip and nitrite height changes [7].
Figure 6: Theory of operation of a cation-exchange membrane suppressor. DBS-H=dodecylbenzenesulfonic acid; DBS-Na =sodium dodecylbenzenesulphonate [7].
The 200 ppm chloride ion sample was tested by passing tube suppressor without a separator column. Water source was the first flowing, and 0.05 M DBS used as scavenger flow. The noise of background conductance reduced from 1.5 mS/cm to 30 µs/cm after adding scavenger. The process conversion of carbonate and sodium bicarbonate to carbonic acid caused lower conductivity. At the same time, sodium chloride was transformed into hydrochloric to make the peak the chloride ion was higher 2.5 times (see Figure 7). The peak height of samples depended on the flow rate of scavenger and flow rate of eluent. The scavenger can be used as any kinds of aqueous acid as soon as using cation-exchange membrane. It was possible the leakage of scavenger into the eluent so the molecular weight of the scavenger should be large [7].
Figure 7: Effect of the tubing suppressor [7].
E) Solid-Phase Chemical Suppressors (SPCSs)
Following suppressors named solid-phase chemical suppressors (SPCSs). This SPCS used disposable pack-bed cartridges. It was protected better than membrane-based suppressor. The cartridge was not expensive so it could be discarded when effecting to signal of peaks. Using these suppressors reduced the maintenance time for regeneration system and avoided the complex post-column reaction system [5].
F) Electrochemically Regenerated Ion Suppressors (ERIS)
Based on the SPCS advance, an electrochemically regenerated ion suppressor (trade mark ERIS) was designed to simplify the way of taking care of suppressors during operation. The ERIS Autosuppressor needed the electrolysis of water to regenerate the solid-phase suppressor cells. Below the reaction of the electrolysis of water in the cells [3];
Anode: 2H2O ---- 4H+ + O2 + 4 e-
Cathode: 2H2O + 2 e- --- 4H+ + O2 + 4 e-
At the both electrodes produced hydrogen ions. One cell was used to suppress the eluent, while other cell was electrochemical suppressor cells helping cycling of the process when the instrument operating analyzed the samples [3].
At the end of each suppressor cell contained electrodes for reaction happening during cation and anion analyses. The hydrogen ion was created at the anode moving to packing sodium from resin converted back the hydrogen form during the electrochemical regeneration. Similarly, the hydroxide ion produced at the cathode was passed through the chloride form resin, changing it back to hydroxide form (see figure 8). By these processes, there were no requirement for regenerant reagents and no due with chemical waste. The problem of ERIS Autosuppressor was the suppressor cell was always finished regenerated before each injection [3].
The ERIS Autosuppressors operated at high pressure, and got along with electroactive eluents and organic containing eluents. The electrochemical regeneration avoided the need for regenerant reagent and equipment. This suppressor had two suppressor cells; one cell used for suppression, and other was regenerated and equilibrated. A switching valve was also need to continuous the operation [3, 8]
For cation analysis used hydrochloric acid as electroactive eluents did not damage the ERIS suppressor cells but it damaged in the membranes. The example of cation (see Figure 9) showed the well separation of cations ion with hydrochloride acid as eluent. Although the sample was run for two weeks without stopping, there was not deterioration in the suppressor cell was detected. However the studied showed the containing of methanol in eluents damaged cell after the electrolysis was done [3].
The advances of electrochemical type suppressor such as: (1) avoid a flowing scavenger solution ;(2) improve the suppression performance; 3) simplify operation procedure: (4) reduce back pressure of suppressor; (5) the suppressor last longer; and (6) decrease cost of suppressor [5].
Figure 8 : suppression and electrochemical regeneration during (a) anion analysis and (b) cation analysis. Cell “A” suppresses the eluent while cell “ B” electrochemically regenerated [3].
Figure 9: Separation of cations using hydrochloric acid eluent. Peak identification: 1=lithium (0.2 ppm), 2=sodium ( 1.5 ppm), 3= ammonium (1.5 ppm), 4= potassium (2.4 ppm), 5=magnesium ( 2.0 ppm), 6=calcium (2.0 ppm). Column universal cation ( 100 mmX 4.6 mm); eluent, 3 mM hydrochloric acid; flow-rate ,1.0 ml/ml; detection, suppressed conductivity. [3]
H) Electrolytic Suppressors (SRS)
New electrolytic suppressor for IC was introduced after electrochemical suppressor. It named the “Self Regeneration Suppressor” or SRS. This new model designed with two platinum electrodes allowed electrodes of water goes through. The electrolytic reaction was generated by a constant controller electric field power. The design of the suppressor (see figure10) included ion-exchange screen to present in the regenerant chambers to help electric current. The ion-exchange membranes identified an eluent chamber that contained another ion-exchange screen to limit dead volume. Two platinum electrodes were inserted in the regenerant chamber; one was in the middle of the hardware shell and a regenerant screen, and other in the middle of the second regenerant screen and the membrane. The present of electrode employed high efficiency and eliminated gases of the waste. In the SRS suppressors showed the neutralization reaction same as in the membrane suppressors [6].
1) Anion Self Regenerating Suppressors( ASRSs)
The SRS used for anion analysis called Anion Self Regenerating Suppressor (ASRS). At the anode hydrogen ion was created passing through the cation-exchange membrane to neutralize the sodium hydroxide or any basis eluent to form water. After neutralizing, the water moved to the detector cell. Positive charge like sodium was attracted to negatively charge cathode. The membrane in the cathode chamber absorbed sodium and released electrogenerated hydroxide ion to maintain electronic neutrality. The gases of hydrogen were generated from cathode and the gases of oxygen were generated from anode combined with the liquid waste of aqueous sodium hydroxide to the waste tubes [6].
2) Cation Self Regenerating Suppressors ( CSRSs)
The SRS used for cation analysis called Cation Self Regenerating Suppressor (CSRS). At the cathode hydroxide ion was generated going to anion-exchange membrane to neutralize the methanesulfonic acid or any acidic eluent to form water. This neutral eluent continued to the detector cell. Methanesulfonate ions was connected to the anode go to cathode membrane reactive with hydronium ions to endure electronic neutrality. Similarly to anion, the waste gases of neutralized processing and methanesulfonic acid went to the waste and vent [6].
The advance of SRS required the low flow-rates for the regenerant water. It used the deionized eluent from the detector cell waste as water source, excellent for reducing waste. SRS was able to handle high sample load and used the high ionic mobile phases. The good thing of an electrolytic suppressor had a low-resistance pathway using ion-exchange sites to flow between the electrodes allowing conductance of the electrical current [6].
There are three self-operational advances in the new electrolytic suppressor such as:
1) Cell effluent recycling: After the eluent went through the separation column, moved to the suppressor where the eluent was neutralized and removed the opposite ions. At this moment, the most the suppressed eluent was deionized ion and very few analyte ions went to the detector cell and came back to the suppressor as a routing. The advance of cell recycling reduced chemical waste cost of operation, and time to maintain suppressors.
2) External water source: the DI water used to increase the flow-rate through the regenerant chambers removing unwanted eluent counterions. This process helped to lower noise and more sensitivity.
3) Chemical regeneration mode: this new suppressor can be used with chemical regenerants as the membrane suppressors [6].
Figure 10: Internal schematic of the Self Regenerating Suppressor [6].
III) No Suppressor Column for Anion Chromatography
1) This system used a conductivity detector for anion chromatography without adding suppressors (see figure 11). It required a special anion-exchange with very low capacity and an eluent had low conductivity.
It required a special anion-exchange resin with very low capacity and an eluent with low conductivity. There were three low-capacity anion-exchange were prepared in lab. [9]
The first one used copolymer (cross-linked polystyrene beads XAD-1) as starting material. This ion-exchange resin had capacity of 0.04 mequiv./g. and its mechanical was stable. Second, 3 g of zinc chloride were added to the mixture of 10 ml of chloromethyl methyl ether, 10 ml of methylene chlorid, 3 ml of nitromethane. The beads were formed after adding water to mixture and they used as ion-exchange resin with capacity of 0.07 mequiv./g. Third, the chloromethylated beads was made from liquefied trimethylamine with capacity 0.007 mequiv./g. An eluent with low conductivity used such as aqueous solution 5.0 *10-4 M of potassium benzoate, potassium phthalate or ammonium o-sulfobenzoate. The resin with low capacity and eluent low conductivity employed the sensitivity of anion chromatography at low concentration. The study showed the resins of lower capacity resulted lower detection limits [9].
The eluent anion was taken by the anion –exchange resin moving anion to be separated to chromatographic column. At low concentration of the eluent, the signals of anions were higher than the eluent background showing a positive peak. If the concentration of eluent greater than anions samples, the peaks of anions showed negative [10].
Figure 11: Separation of 4.8 ppm of fluoride, 5.1 ppm of chloride and 26.0 ppm of bromide. Resin, XAD-1, 0.04 mequiv./g; Eluent, 6.5* 10-4 M potassium benzoate, pH=4.6 [9].
(SO42-, Cl- , NO2- , Br- , and NO3-) were found in both samples with flowing concentration 42.1, 78.3, 0.81, 0.36 and 10.2 μM in snow and 20.1, 38.7, 0.41, 0.17, and 5.2 M μmin rainwater. The retention time of samples were changed depending on the concentration of eluent [14].
The liquids samples from a drill stem test (DST) containing reservoir brine was found 5.5 ppm lithium, 58170 ppm sodium, 147 ppm ammonium and 1353 ppm potassium. The second DST sample containing reservoir brine and potassium chloride-based drilling mud was found 3.2 ppm lithium, 33570 ppm sodium, 65 ppm ammonium and 7980 ppm potassium. The study also identifies that hydrochloric acid does not elute cation earth metals [15].
Alkanli an alkaline earth ions are analyzed by IC to be able separated. The samples contain 1.0 ppm lithium, 9700 ppm sodium, 98 ppm ammonium, 63 ppm potassium, 85 ppm magnesium, 152 calcium, 17.6 ppm strontium, 1.0 ppm barium. This method used an IonPac CS12A as the separation column and H2SO4 as eluent. This is very useful method to analyze cation ions [16].
IV. In Conclusion:
Ion Chromatography can analyze with or without suppressors presentment. However, the report showed that suppressors have better sensitivity and more dynamic of peaks ions than without suppressors. The growing of ion chromatography instrument developed in commercially a decade ago. IC is a mature instrument with good sensitivity. It is able to analysis both cations and anions and easy to maintain [13]. The studiers continue research and development to respond to the analytical challenges of ion chromatography [1]. Ion Chromatography is advance instruments for analyzing earth metals and useful to determine environmentally waters [4].
References:
1) Chaga, G. Twenty-five years of immobilized metal ion affinity chromatography: past, present and future. J. Biochem. Biophys. Methods 2001, 49, 313-334.DOI
2) Small, H.; Stevens, T. S.; Bauman, W. C. Novel ion exchange chromatographic method using conductimetric detection. Anal. Chem. 1975, DOI
3) SaariNordhaus, R.; Anderson, J. Electrochemically regenerated ion suppressors, new suppressors for ion chromatography. Journal of Chromatography a 1997, 782, 75-79.DOI
4) Stevens, T.; Davis, J.; Small, H. Hollow Fiber Ion-Exchange Suppressor for Ion Chromatography. Anal. Chem. 1981, 53, 1488-1492.DOI
5) TIAN, Z.; HU, R.; LIN, H.; WU, J. High-Performance Electrochemical Suppressor for Ion Chromatography. J. Chromatogr. 1988, 439, 159-163.DOI
6) Rabin, S.; Stillian, J.; Barreto, V.; Friedman, K.; Toofan, M. New Membrane-Based Electrolytic Suppressor Device for Suppressed Conductivity Detection in Ion Chromatography. J. Chromatogr. 1993, 640, 97-109.DOI
7) Hanaoka, Y.; Murayama, T.; Muramoto, S.; Matsuura, T.; Nanba, A. Ion Chromatography with an Ion-Exchange Membrane Suppressor. J. Chromatogr. 1982, 239, 537-548.DOI
8) Saari-Nordhaus, R.; Anderson, J. Recent advances in ion chromatography suppressor improve anion separation and detection. Journal of Chromatography a 2002, 956, 15-22 DOI
(9) Gjerde, D.; Schmuckler, G.; Fritz, J. Anion Chromatography with Low-Conductivity Eluents .2. J. Chromatogr. 1980, 187, 35-45. DOI
(10)Gjerde, D.; Schmuckler, G.; Fritz, J. Anion Chromatography with Low-Conductivity Eluents .2. J. Chromatogr. 1979, , 509-519. DOI
(11) Rapsomnikis, S.; Harrison, R. Optimization of Single-Column Anion Chromatography with Indirect Ultraviolet Photometric and Fluorimetric Detection. Anal. Chim. Acta 1987, 199, 41-47 DOI
(12) Noble, D. Ion Chromatography, Anal. Chem., 67 (1995), p. 205A DOI
(13) Kadnar, R. Determination of alkali and alkaline earth metals in oilfield waters by ion chromatography. Journal of Chromatography a 1998, 804, 217-221. DOI
(14) Hu, W.; Tanaka, K.; Haddad, P.; Hasebe, K. Suppressed electrostatic ion chromatography with tetraborate as eluent and its application to the determination of inorganic anions in snow and rainwater. Journal of Chromatography a 2000, 884, 161-165. DOI
(15) Morales, J.; Demedina, H.; Denava, M.; Velasquez, H.; Santana, M. Determination of Organic-Acids by Ion Chromatography in Rain Water in the State of Zulia, Venezuela. Journal of Chromatography a 1994, 671, 193-196.DOI
(16) Maurino, V.; Minero, C. Cyanuric acid-based eluent for suppressed anion chromatography. Anal. Chem. 1997, 69, 3333-3338.DOI